A tipped cigarette or other tipped smoking rod comprises a tobacco rod and a filter tip secured adjacent to one end of the tobacco rod. The filter tip comprises a filter body, that may be of homogeneous or heterogeneous construction, and that is enclosed within a sheet material, generally in cylindrical form, that is often termed a "plugwrap" material. The tip is held to the end of the tobacco rod by an outer wrapping material that is wrapped around the tip and the end of the tobacco rod, and that is often termed a "tipping overwrap" material, or a "cork" material. The tipping overwrap is generally coloured brown.
It is often required that the filter should contribute a significant amount of ventilation to the smoke stream being drawn through the filter and accordingly the wrapped laminate created by the tipping overwrap and the plugwrap must be such as to permit the desired degree of airflow through the laminate and into the filter body. If both materials have very low permeability then there will be inadequate ventilation. If both are highly permeable (for instance up to about 600 Coresta) there will be too much ventilation. It has therefore been accepted that it is desirable for the tipping overwrap to be of substantially impermeable material that is perforated to provide ventilation, and it is then necessary for the plugwrap to be permeable underneath the perforations, so as to give the desired ventilation into the filter body.
The use of a permeable plugwrap, for instance having a permeability in the range 200 to 650 Coresta, is therefore common but does incur the known result that the smoke quality is rather bland. Some smokers would prefer to have a more stringent smoke quality from a filter cigarette. It is known that this can be achieved if the plugwrap material is substantially impermeable but is provided with perforations in registration with the perforations through the tipping overwrap. Unfortunately it is extremely difficult to provide in an economic manner a reliable degree of registration of the two sets of perforations, and any variation in registration will inevitably result in variation in ventilation. It should be noted that it is important that the degree of ventilation is substantially uniform from one smoking rod to another as otherwise the smoke qualities will vary from one rod to another.
Registration problems are eliminated if the tipping overwrap and plugwrap are perforated simultaneously, after assembly of the smoking rod. This is described in, for instance, U.S. Pat. No. 4,564,030 and EP 255,114. Unfortunately it is difficult to perform in practice. It is necessary for there to be a large number (e.g., at least 10) peforations distributed around the smoking rod. The most practicable way of achieving these perforations is by rotating the rod as it travels longitudinally past the perforating apparatus. The preferred perforating apparatus is a laser. Unfortunately suitable lasers are expensive, cannot be fitted to all types of cigarette making machines and their use results in a loss of cigarette making efficiency. Also the laser itself is under-utilised because the cigarette making machine has to run at a linear speed that is much less than the linear speed at which the laser could give satisfactory perforations. Another source of inefficiency is that a significant number of cigarette rods are liable to be broken during the high speed rotation of them.
One possibility that we have considered is the provision of perforations uniformly distributed over the entire plugwrap material. Unfortunately this proves impracticable. If the perforations are sufficiently close to one another to try to ensure a sufficient degree of registration of the two sets of perforations to give the necessary ventilation, the extent of perforation of the plugwrap is so great that the plugwrap has insufficient longitudinal strength to withstand the forces to which it is subjected during the manufacture of the filter rod and subsequently the smoking rod. However there are unsatisfactory variations in the degree of registration and if the overall perforation is reduced to promote longitudinal strength, then the degree of ventilation becomes even more variable.
In GB 2,105,171 it is proposed to provide a plugwrap which has a regular array of apertures that are arranged in staggered transverse rows (i.e. perpendicular to the length direction of the plugwrap) and staggered lengthwise columns whereby the total length of void traversed by any two transverse lines is the same. The plugwrap apertures have a width about 2.5 mm. The tipping overwrap has perforations that are small relative to the plugwrap perforations, the tipping overwrap perforation typically being 0.01 to 0.5 mm in diameter.
There is no suggestion where the said transverse lines should be positioned and this system has serious disadvantages. The very large perforations in the plugwrap material weaken it and this can cause handling problems, especially when the filter body is a heterogeneous construction having voids such as shown in FIG. 3 of GB 2,105,171. Another disadvantage is that the very large perforations in the plugwrap material allow the stained filter body to be readily visible through perforations in the tipping overwrap, and this can be highly undesirable.
Another, and fundamental, problem with the arrangement is that it does not suggest how to provide uniformity of ventilation from one filter tip to another. Uniformity would perhaps be available if the perforations in the tipping overwrap were replaced by continuous transverse slots, since the same area of perforations would then be exposed irrespective of the longitudinal and transverse positioning of the tipping overwrap with respect to the plugwrap. In reality however the tipping overwrap perforations have to be discrete apertures and the extent to which they overlap the large perforations will depend upon chance. For instance if the perforations in the transverse row had a pitch identical with the pitch between the longitudinally extending rows in the plugwrap, the extent of overlap could range from 100 to 0%, depending upon the radial positioning of the tipping overwrap relative to the plugwrap.
A particular problem arises in ventilated filter tips in that it is often preferred that the perforations in the tipping overwrap should be relatively large so that they are visible to the naked eye, and if the perforations through the plugwrap are also relatively large then the degree of ventilation would be much too high. Accordingly these smoking rods require that the perforations through the plugwrap should be smaller than the perforations through the tipping overwrap, and this creates additional difficulties both when the plugwrap is to be perforated in a pre-assembled smoking rod and when it is necessary to achieve uniform registration of previously perforated tipping overwrap and plugwrap.
The present situation therefore is that it is known that improved smoke characteristics can be achieved if the plugwrap and tipping overwrap are both perforated but otherwise substantially impermeable materials, but that there is no satisfactory method of making such filters. Either the perforations are made simultaneously by machinery that is very expensive and inefficient, or the materials are perforated previously and a consistent degree of ventilation is not obtained.